Regex is not working

Rahul Gupta has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

I am trying to substitute values in query but things are not working for me.

Here is my code

my @array_1 = ("\$Id_run_query=1", "\$name_run_query=xyz", "db_query_r
+un_query=select * from meter where metetid=\$Id and metername=\$name"
+);
my $query;
foreach(@array_1){
    if ($_ =~ m/^db_query_run_query=(.*)/i){$query = $1;print "$1\n";}
}
foreach(@array_1){
    if ($_ =~ m/^(\$\S+)=(.*)/i){
        my $replace = $2;
        my $value = shift @{ [ split( /_/, $_ ) ] };
        print "$value\n";
        chomp($value);
       # print "$query\n";
        $query =~ s/$value/$replace/i;
    }
}
print "===>$query\n";
[download]

the expected output is :

"select * from meter where metetid=1 and metername=xyz"

but i am geeting this:

"select * from meter where metetid=$Id and metername=$name"

Thanks in advance

Comment on Regex is not working Select or Download Code

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Re: Regex is not working by moritz (Cardinal) on Oct 03, 2013 at 05:21 UTC
`$query =~ s/$value/$replace/i;` The problem is that `$value` contains something like `$Id`, and the $ when interpreted as a regex means "match the end of the string", not "match a dollar sign". You can fix that by writing `s/\Q$value\E/$replace/i;` [download] instead. Fwiw this is how I'd solve the same task: `my @array_1 = ("\$Id_run_query=1", "\$name_run_query=xyz", "db_query_r +un_query=select * from meter where metetid=\$Id and metername=\$name" +); my $query; my %replace; foreach(@array_1){ if (m/^db_query_run_query=(.)/i){ $query = $1 } elsif (/(\$\S+)_run_query=(.)/) { $replace{$1} = $2; } } my $replace_re = join '\|', map quotemeta, keys %replace; $query =~ s/($replace_re)/$replace{$1}/ig; print "===>$query\n";` [download] The call to quotemeta does the same as `\Q...\E` in the regex. Perl 6 - the future is here, just unevenly distributed	[reply] [d/l] [select]
Re^2: Regex is not working by Rahul Gupta (Sexton) on Oct 03, 2013 at 05:28 UTC
Thanks it worked for me:-)	[reply]
Re: Regex is not working by CountZero (Bishop) on Oct 03, 2013 at 16:06 UTC
If this query is to be used in a DBI environment to get data out of the database, wouldn't it be much safer (and probably easier too) to simply use "?" placeholders? CountZero A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James My blog: Imperial Deltronics	[reply]
Re: Regex is not working by kcott (Archbishop) on Oct 03, 2013 at 06:18 UTC
G'day Rahul Gupta, Your underlying problem is that the value of `$value` in `$query =~ s/$value/$replace/i` is being interpolated: ~~`$Id` and `$name` evaluate to zero-length strings so `$query =~ s/$value/$replace/i` is equivalent to `$query =~ s//$replace/i` (i.e. no substitutions occur).~~ That was a poor guess on my part; see moritz' explanation instead; the fixes, however, are identical. To fix, use `$query =~ s/\Q$value\E/$replace/i;` [download] -- Ken	[reply] [d/l] [select]